Light echoes from ancient supernovae in the Large Magellanic Cloud

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Echoes from Ancient Supernovae in the Large Magellanic Cloud

Armin Rest1, Nicholas B. Suntzeff1, Knut Olsen1, Jose Luis Prieto2, R. Chris

Smith1, Douglas L. Welch3, Andrew Becker4, Marcel Bergmann5, Alejandro

Clocchiatti6, Kem Cook7, Arti Garg8, Mark Huber7, Gajus Miknaitis4, Dante

Minniti6, Sergei Nikolaev7, & Christopher Stubbs8

1Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory9,

La Serena, Chile

2Dept. Astronomy, Ohio State University, Columbus, OH 43210 USA

3Dept. Physics & Astronomy, McMaster University, Hamilton, ON, L8S 4M1, Canada

4Dept. Astronomy, University of Washington, Seattle 98195 USA

5Gemini Observatory9, La Serena, Chile

6Dept. Astronomia y Astrofisica, Pontifica Universidad Católica de Chile, Santiago,

Chile

7Lawrence Livermore National Laboratory, Livermore, CA 94550 USA

8Dept. of Physics and Harvard/Smithsonian Center for Astrophysics, Harvard

University, Cambridge, MA 02138 USA

9 Based on observations obtained at NOAO, operated by the Association of Universities

for Research in Astronomy, Inc. (AURA) under cooperative agreement with the NSF.

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In principle, the light from historical supernovae could still be visible as scattered-

light echoes even centuries later1-6. However, while echoes have been discovered

around some nearby extragalactic supernovae well after the explosion7-13, targeted

searches have not recovered any echoes in the regions of historical Galactic

supernovae14-16. The discovery of echoes can allow us to pinpoint the supernova

event both in position and age and, most importantly, allow us to acquire spectra

of the echo light to type the supernova centuries after the direct light from the

explosion first reached the Earth. Here we report on the discovery of three faint

new variable surface brightness complexes with high apparent proper motion

pointing back to well-defined positions in the Large Magellanic Cloud (LMC).

These positions correspond to three of the six smallest (and likely youngest)

previously catalogued supernova remnants, and are believed to be due to

thermonuclear (Type Ia) supernovae17. Using the distance and proper motions of

these echo arcs, we estimate ages of 610 and 410 yr for the echoes #2 and #3.

As part of the SuperMACHO microlensing survey, we have been monitoring the

central portion of the LMC every other night for three months each year over the last

four years (2001-4). Using an automated pipeline, we subtract point-spread-function

matched template images from the recent epoch images. The resulting difference images

are remarkably clean of the constant stellar background and are ideal for searching for

variable objects.

The well-known echo of SN1987A shown in Figure 1 was trivial to recover in the

difference images with our pipeline. The high apparent motion of the echoes, often

superluminal, allows simple detection in difference images. To search for very faint

echoes, we have examined by eye all the variable objects discovered by our automatic

pipeline. We found a number of very faint linear structures that had high proper motions

with vector directions inconsistent with the 1987A echo. For each structure, we

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estimated a vector direction as shown in Figure 2. Figure 3 shows the echo vectors

extrapolated backward in time pointing to three well-defined positions as the origins of

the echo complexes. The origins of the four echo complexes are listed in Table 1. The

three unidentified echo origins correspond within arcminutes of the positions of known

supernova remnants (SNR)18 and also correspond to three of the six youngest SNRs17.

These three SNRs are precisely the three that are classified as likely Type Ia events

based on the X-ray emission spectra.

Given the positional match with young SNRs and the high apparent proper

motions of the variable diffuse light, we conclude that these newly detected structures

are likely to be scattered light echoes from Type Ia supernovae in the LMC. Planned

spectroscopy of the brightest knots in the three echo complexes should allow us to

determine the type of the supernovae and confirm the classifications from the X-ray

studies.

The theory of supernova light echoes (whereby we mean the actual scattered light

echo rather than fluorescence or dust re-radiation) predicts that light echoes can be seen

even centuries after the first arrival of light from the explosion. Using a differential

form of equation 7 for surface brightness19, we find for two different supernovae:

Σ2 = Σ1 + (V2_SN - V1_SN) -2.5log10(r1 t1/ (r2 t2)) -2.5log10(Φ2/Φ1)

where Σ is the echo surface brightness, V_SN is the supernova magnitude at maximum, r

is the echo to supernova distance, t is the time between explosion and echo observation,

and Φ is the Henyey-Greenstein phase function. Here we assume that the SN light pulse

duration is the same for the two supernovae, and that the composition, density, and

thickness of the dust sheets producing the echoes are identical. We also calculate the

Φ function with forward scattering (g=0.6), and only include the angular terms. Scaling

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from the brightest echo knot of SN1987A at 19.3 mag arcsec-2, we find that a 500 year

old Type Ia SN that exploded 150pc behind a face-on dust sheet would produce a light

echo with a surface brightness of 22.5 mag arcsec2 at an angular distance of 0.29º

(250pc radial distance from the SN) assuming a Type Ia supernova was 3.5mag brighter

than SN1987A. At 1000 years, the echo would be 24 mag arcsec2 at an angular distance

of 0.5º or 420pc from the explosion site. These surface brightness estimates are

consistent with the echoes discovered here.

Supernova light echoes can be used to measure the structure and nature of the

interstellar medium4, 20, 21 and, in principle, can be used to measure geometric

distances22. The geometric relationship is ρ = (ct(2z + ct))1/ 2 where ρ is the apparent

projected radius of the light echo on the sky, z is the distance from the supernova to the

dust sheet, and t is the time since peak brightness of the source. Given the known

distance to the LMC and time of explosion, the echoes in Figure 1 can be used to map

out the structure of the dust23.

What are the ages of the supernovae producing these echoes? A Type Ia SN in the

LMC would reach an apparent magnitude of V~ −0.5 and would be the second or third

brightest star in the southern sky for a few weeks. Lower limits on the supernova ages

can be set from the absence of reported bright supernovae since the establishment of the

Royal Observatory at the Cape in 1820. An independent lower limit of 300 yr can be

derived from the sizes of these SN remnants assuming an unrealistic large constant

shock velocity of 10,000 km s-1.

We can use the apparent expansion velocity to crudely measure the ages of the

supernova echoes. A simple differentiation of the formula above gives v=c(z+ct)/ρ

where v is the expansion velocity assuming the dust plane is perpendicular to the line of

sight and c is the speed of light. Solving the two equations simultaneously, we find the

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age for echo 2 is 600 ± 200 yr with the dust 570 ± 180pc in front of the SN based on 9

arcs, and for echo 3, an age of 400 ± 120 yr with the dust 340 ± 160pc in front of the SN

based on 6 arcs. Echo 4 only had one arc with a superluminal velocity, giving an age of

860 yr. The alternative solutions to the equations gave ages greater than 2500 yr, which

are excluded based on upper limits of less than 1000 to 1500 yr from the optical and X-

ray observations24. As a check on this technique, we measured an age for the SN1987A

echo of 15.9 ± 1.4 yr from 39 echo arc positions, which is consistent with the age of

1987A at the epoch of observation of 14.8 yr.

The uncertainties quoted above are the standard deviation of estimates from the

different arcs. The uncertainties in the proper motions, which are typically 0.1 arcsec yr-

1, propagate to age uncertainties of less than 50 yr. The largest uncertainty in the age

estimates comes from the unknown inclinations of the dust sheets (assumed to be zero,

or perpendicular to the line of sight). Allowing for inclinations leaves the upper limit on

the ages unbounded, but lower limits can still be derived. If the dust sheets have

inclinations of less than 60 degrees, we find lower limits of 400 yr, 250 yr, ad 380 yr for

the ages of echo 2, 3, and 4 respectively.

Also intriguing is the opportunity they provide for directly observing the spectral

light from the historical supernovae themselves as Zwicky25 suggested in 1940. Precise

image subtraction techniques on nearby galaxies and in our own Galaxy with modern

digital images can reach much fainter surface brightness limits than the early

photographic surveys and allow us to find echoes from supernovae as old as 1000 years

or more. With the discovery of a bright echo knot, we might be able today take a

spectrum, representing the time average of the light at maximum, of the Tycho, Kepler,

SN1006, or Cas A supernova. As an example, for a dust sheet 400pc in front of the

Tycho SN with Vmax=-6.5, a distance of 3.1kpc, and knots of densities similar to the

highest density sheets near SN1987A, the surface brightness would be 22 mag arcsec-2.

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The arc would be at 6.5º from the Tycho SNR and would move at 30" yr-1. Scaling the

typical echo width from the LMC, the Galactic echo would be ~30" wide. A survey

utilizing digital subtraction over an area of 100 sq-degree could be able to recover these

moving arcs.

Style tag for received and accepted dates (omit if these are unknown).

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Acknowledgements C.S. thanks the National Science Foundation, the McDonnell Foundation, and

Harvard University for their support of the SuperMACHO project. D.W. acknowledges support from the

Natural Sciences and Engineering Research Council of Canada (NSERC). The work of K.C., M.H. and

S.N. was performed under the auspices of the U.S. Department of Energy, National Nuclear Security

Administration by the University of California, Lawrence Livermore National. A.C. acknowledges

support from FONDECYT. DM was partially supported by FONDAP. J.P. was funded by the OSU

Astronomy Department Fellowship.

Compelling interests statement The authors declare that they have no compelling financial interests.

Correspondence and requests for materials should be addressed to N.S. (e-mail:

[email protected]).).

Figure 1. The light echoes from SN 1987A. The data, taken at the CTIO 4m

Blanco telescope with the MOSAIC imager in the VR filter, were used to make

this difference image with epoch 2004.97 minus 2001.95 data, representing

17.8 and 14.8 years after the explosion. Our SuperMACHO survey covers 24

sq-degrees in 68 pointings in an approximate rectangle 3.7° by 6.6° aligned with

the LMC bar. The images are taken through our custom “VR” filter (λc=625nm,

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∆λ=220nm) with exposure times of 60s to 200s, depending on the stellar

densities. The field is 13.8’ by 18.4’ with N up and E left. White represents flux

enhancements in the 2004 image and black in the 2001 image. Faint echo arcs

can be seen as far out as 6.6’ and 7.3’ from the explosion site, or 0.9 and

1.1kpc in front of SN 1987A. The VR surface brightness varies from 19.8 to a

limit of ~24 mag arcsec-2 with one knot as bright as 19.3 mag arcsec-2. The

widths of the echoes are resolved, and typically ~2.5" across.

Figure 2. Arcs of light echoes in the Large Magellanic Clouds from previously

unseen supernovae. Panel 1 (upper left) shows the unsubtracted (template)

image which includes the cluster Hodge 243. Panel 2 (upper right) shows how

cleanly the field subtracts with data taken 50d earlier. The next three panels

show the echo motion 1, 2, and 3 years after the template date. White

represents positive flux in the present epoch image and black in the template

image. The vector motions are plotted in Panel 6 (lower right). Each echo is fit

with a straight line (red). The apparent proper motion is given by the yellow

vector and extrapolated backwards (blue). The size of the yellow vector is

proportional to the length of the echo segment fit. Saturated stars are masked

out with grey circles. A number of faint variable stars appear as black or white

spots. The vector was defined to be perpendicular to a linear fit to an echo

segment, with the direction given by the proper motion. Typical proper motions

range from 0.5-2.4” yr-1 which, at the angular scale of the LMC of 0.77 light-year

arcsec-1 makes many of these structures have apparent superluminal velocities.

The surface brightness ranges from 22.3 mag arcsec-2 down to our limit of

detection at 24 mag arcsec-2. These echoes are located in echo complex #2, at

RA, Dec=(05:16:06,-69:17:07, J2000). Each panel is 80” on a side with N up

and E to the left.

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Figure 3. A plot of the light echo vectors in the LMC. The vectors have the

same meaning as in Figure 2. The centres of the echo complexes are indicated

by yellow circles. The lengths of the yellow vectors are 100x the length of the

echo arc. The source on the left marked with a star is SN1987A. The green

circles are the location of historical novae, and the red circles are the supernova

remnant locations25. Evidently, the three unknown echo complexes point to

three catalogued supernova remnants. We have estimated the position of the

crossing point of the vectors by calculating the crossings of all pairs of vectors

in each group excluding any echo pair with a separation of less than 10".

Table 1: Positions of Supernova Echo Origins in the LMC

Echo complex RA dec position error δr SNR name

1 05:35:30 -69:16 0.1 0.2 SN1987A

2 05:19:14 -69:04 1 2.5 0519-69.0

3 05:11:17 -67:31 1 10.0 0509-67.5

4 05:09:19 -68:42 2 2.3 0509-68.7 (N103B)

Position errors, based on the intersection of the echo vectors, are given in arcminutes. δr, the

distance between the tabulated echo origin and SNR, is given in arcminutes. Coordinates are

equinox J2000. The error in the centroid was estimated from the averaged vector crossings.